Compressors, e.g., turbochargers, are used in motor vehicles for supercharging internal combustion engines to increase power. Turbochargers therefore have a high-speed compressor impeller having blades which draw air in from the environment and feed it at a higher pressure into an air system of the internal combustion engine.
The compressor impeller is driven by a turbine located in the exhaust line of the internal combustion engine and drives the compressor impeller as a function of the exhaust gas enthalpy. The air supplied at a higher pressure in the air system of the internal combustion engine results in a greater charge in the combustion chamber and thus in greater torque and power at the same volume.
Exhaust gas turbocharging is being used increasingly today to increase power and also to reduce fuel consumption by downsizing. In the latter case, the increased power is utilized to reduce the displacement and thus reduce the size of the entire internal combustion engine and lengthen the transmission. The reduction in fuel consumption results on the one hand from the shift in the operating points of the engine into ranges of more efficient combustion and on the other hand from a reduced friction due to the compact design and the reduced engine speed.
It is characteristic of the exhaust gas turbocharger that the rotor assembly having the turbine, the compressor and the shaft connecting the turbine and the compressor impeller is able to rotate completely unbraked. The rotational speed of the rotor assembly is obtained as a function of the enthalpy of the exhaust gas mass flow and the power absorbed by the compressed air. The rotational speed is so high at the operating point having a high compression ratio and/or a high volume flow that the centrifugal forces at the circumference of the compressor impeller may result in deformation of the blades of the compressor impeller. The compressor impeller and other high-speed components of the turbocharger are therefore highly susceptible to wear and material fatigue, so the lifetime of the turbocharger depends on its operating conditions to a great extent.
If the turbocharger is operated near the pumping limit or in the pumping state, the blades of the compressor impeller may be excited to mechanical vibrations, which may cause permanent damage to the compressor impeller if the load is too high or if there is a continuous load. The vibrations of the blades cause material fatigue more rapidly and ultimately cause fatigue fractures, so that the lifetime of the turbocharger is greatly impaired when pumping states occur. Frequent operation of a turbocharger at an excessive rotational speed, at which the outer ends of the compressor blades are able to achieve very high velocities, results in a definitely increased material fatigue and thus a reduced lifetime.
Manipulations involving the engine system in particular in tuning the vehicle to increase power may result in the turbocharger frequently being operated at an excessive speed. This is an abuse of the turbocharger because it significantly shortens its lifetime. If the turbocharger is defective, the manufacturer incurs high costs, it being difficult to prove abuse.
In addition, wear phenomena occur on the bearings of the turbocharger during operation of the turbocharger, resulting in changes in the uniformity of rotation over the lifetime of the turbocharger. The rate of wear on the individual components of the turbocharger depends on the operating states during the operating time of the turbocharger. The degree of wear determines the remaining lifetime.
Since damage to the compressor impeller should be prevented, it is advisable to monitor the rotational speed of the compressor impeller and it is necessary to ensure that the rotational speed range in which the compressor impeller is damaged is never reached. In traditional internal combustion engines for passenger vehicles, the rotational speed of the compressor impeller is not usually determined directly but instead is derived from thermodynamic variables, typically the pressure ratio and the volume flow through the compressor. Operating characteristics of the compressor, which are usually subject to scattering within a series, are used here.
Furthermore, the detection or modeling of the variables (pressure, temperature of the mass flow and the like) required for this purpose is itself subject to tolerance. To take this into account, a safety margin from the maximum allowed rotational speed is usually defined, but this means that the potential of the turbocharger is never fully utilized. Furthermore, the turbocharger must be designed with larger dimensions so that it has an inferior response in low power ranges, in particular at low engine speeds. The goal must therefore be to operate the turbocharger as close to its limit speed as possible.
On the other hand, other defects in the engine system may result in failure of the indirect determination of rotational speed as described above. For example, if a rotational speed of the turbocharger is determined that is too low when a leak occurs downstream from the compressor, thereby increasing the volume flow, but the engine control cannot detect this accurately or at all, depending on the sensor configuration. In addition, blockage of the air filter may result in a pressure drop upstream from the compressor and thus an increased compression ratio at the same air mass flow and thus result in an increased rotational speed of the turbocharger. It is therefore advisable to measure the rotational speed of the turbocharger directly to avoid the disadvantages described above.
Methods are already known which are based in principle on detection of a rotational speed of the compressor impeller by measuring the frequency at which the blades of the compressor impeller pass by a sensor element. The sensor element is situated close to the blades and detects the passing blades by detecting an inductive change in a resistance or a current flow as a response to the interruption in a magnetic field of a permanent magnet as the blades of the turbocharger pass by it.
This method of ascertaining the rotational speed has the disadvantage that due to the high temperatures in the compressor, the available sensors are not suitable because they may be sensitive to temperature. Furthermore, the available installation space on the compressor housing is very limited, so that installation of a corresponding sensor close to the blades of the compressor impeller is limited in particular in small turbochargers. The adjustment when using the measurement principles mentioned above is also demanding because under some circumstances there are requirements concerning the positioning of the sensor in the submillimeter range.
Two-stage turbocharging devices are increasingly being used in engine systems. Two turbocharging units (compressor stages) are situated here in series, or in parallel to one another, often having different compressor capacities. The compressor stage having the lower compressor capacity has a lower inertia and is used to take over the charging at low engine loads, e.g., at the start of acceleration of a vehicle. Therefore, because of its faster response time, a rapid buildup of the charging pressure is made possible and it is possible to respond rapidly to rapid changes in torque demand by the internal combustion engine. Meanwhile, the compressor stage having the higher compressor capacity may be switched to inactive mode by a bypass valve. Then the compressor stage having the high compressor capacity subsequently assumes the function of providing the corresponding high air mass flow with an increase in engine load. The compressor stage having the low compressor capacity is then bypassed by another bypass valve and therefore switched to inactive mode. Controlling the bypass valve, which is necessary for adjusting the corresponding compressor capacities, requires knowledge of the rotational speed of the individual turbocharging units of the compressor stages. However, providing individual rotational speed measurements is complex because of the limited installation space and because of the turbocharging devices being situated close to one another locally in such a multistage charging system.